Many studies report sex differences in pain, with women more sensitive to experimental pain and more vulnerable to many clinical pain syndromes. While the biological bases for these differences are unknown, the recent finding that women activate endogenous pain inhibitory systems to a lesser degree than men in response to heterotopic noxious conditioning stimulation (HNCS) suggests that sex differences in pain may be partly attributable to differential activation of endogenous analgesic systems. Even so, multiple neural systems mediate endogenous analgesia and these systems can be activated by a wide variety of factors, including pain, stress, and distraction. Thus, it is unclear whether sex differences in HNCS-evoked pain inhibition arise from differential effects of pain, stress, or distraction on endogenous pain modulation. The major objectives of this project are to explore the factors that contribute to HNCS-evoked pain inhibition in humans, and identify sex differences in the contributions of these factors to endogenous pain inhibition. To accomplish these aims, psychophysical tests will be conducted in healthy human volunteers, using separate distracting, stressful, and painful conditioning protocols to activate endogenous analgesic systems.